In an optical communication network, optical signals having a plurality of optical channels at individual wavelengths (i.e., channels), are transmitted from one location to another, typically through a length of optical fiber. Various types of optical switches are available which allow switching of optical signals from one optical fiber to another. A wavelength-selective switch (WSS), allows reconfigurable wavelength-dependent switching, that is, it allows certain wavelength channels to be switched from a first optical fiber to a second optical fiber while letting the other wavelength channels propagate in the first optical fiber, or it allows certain wavelength channels to be switched to a third optical fiber. An optical network architecture based on wavelength-selective optical switching has many attractive features due to the ability to automatically create or re-route optical paths of individual wavelength channels. It accelerates service deployment, accelerates rerouting around points of failure of an optical network, and reduces capital and operating expenses for a service provider, as well as creating a future-proof topology of the network.
Different switching architectures offer different degrees of flexibility with respect to the allocation and routing of individual wavelengths. FIG. 1 shows a functional block diagram of an N×M optical switch 12 illustrating one routing function that is sometimes desired. The N×M optical switch 10 has N input ports 101, 102, 103 . . . 10N (“10”) and M output ports 151, 152, 153, . . . 15M (“15”). The input ports 10 are WDM ports that are able to receive a wavelength division multiplexed (WDM) signal that includes multiple wavelengths or channels. As explained below, the routing function that is desired is sometimes referred to as contentionless routing.
In contentionless routing, if, as shown in FIG. 1, a signal 1 of wavelength λ1 received on one input port is sent to a given output port and signal 2 of the same wavelength λ1 (but with different data content) is received on a different input port, then signal 2 can be sent to any output port except to the output port to which signal is sent. In other words, optical switch 12 is able to establish a connection from one input port to one output at one wavelength without preventing a second connection from being established at that same wavelength between a second input port and a second output port.
Wavelength selective switches having the functionality shown in FIG. 1 have been demonstrated. Such wavelength selective switches have a relatively small number of input and output ports and are not easily scaled up to provide significantly more ports. For example, U.S. Patent Publication No. 2014/0294346 to Neilson et al. shows one WSS that employs an LCOS array as the switching element. In this device each WDM signal received on an input port is imaged and spectrally dispersed along a separate row of the LCOS array. Because of its design, this WSS requires M physically separate rows to be allocated on the LCOS array to provide the functionality of an N×M switch, where M>N. This becomes impractical for devices with a relatively large number of ports because of the large amount of space that needs to be allocated on the surface of the LCOS array.